NTA, her wedding and her rules but with this family there will be consequences. Cousins behaviour could have been handled long ago but seems they enable it instead. Just cause you are disabled doesn't mean you can't learn and change your behaviour but instead they encourage his bad behaviour.

NTA. A 2 year old can understand the basic premise of not touching people without their consent. If the cousin has the intellectual capacity of a 7 year old he should have been taught, in an age appropriate manner, that his behavior was inappropriate. This is a failing of his parents and your family for treating him like his disability instead of someone capable of learning and respecting others. And before I get called 'ablest', my cousin has an intellectual disability and is assessed as functioning not more than 10 years old. He was taught, patiently and consistently, to respect peoples boundaries. And he does. We respect him by treating him like any other family member capable of learning boundaries, and not like he is nothing more than his disability. If your family refuses to teach him personal boundaries then you should enforce your personal boundaries in whatever manner you consider appropriate.